# = PStore -- Transactional File Storage for Ruby Objects
#
# pstore.rb -
# originally by matz
# documentation by Kev Jackson and James Edward Gray II
#
# See PStore for documentation.
require "fileutils"
require "digest/md5"
#
# PStore implements a file based persistence mechanism based on a Hash. User
# code can store hierarchies of Ruby objects (values) into the data store file
# by name (keys). An object hierarchy may be just a single object. User code
# may later read values back from the data store or even update data, as needed.
#
# The transactional behavior ensures that any changes succeed or fail together.
# This can be used to ensure that the data store is not left in a transitory
# state, where some values were updated but others were not.
#
# Behind the scenes, Ruby objects are stored to the data store file with
# Marshal. That carries the usual limitations. Proc objects cannot be
# marshalled, for example.
#
# == Usage example:
#
# require "pstore"
#
# # a mock wiki object...
# class WikiPage
# def initialize( page_name, author, contents )
# @page_name = page_name
# @revisions = Array.new
#
# add_revision(author, contents)
# end
#
# attr_reader :page_name
#
# def add_revision( author, contents )
# @revisions << { :created => Time.now,
# :author => author,
# :contents => contents }
# end
#
# def wiki_page_references
# [@page_name] + @revisions.last[:contents].scan(/\b(?:[A-Z]+[a-z]+){2,}/)
# end
#
# # ...
# end
#
# # create a new page...
# home_page = WikiPage.new( "HomePage", "James Edward Gray II",
# "A page about the JoysOfDocumentation..." )
#
# # then we want to update page data and the index together, or not at all...
# wiki = PStore.new("wiki_pages.pstore")
# wiki.transaction do # begin transaction; do all of this or none of it
# # store page...
# wiki[home_page.page_name] = home_page
# # ensure that an index has been created...
# wiki[:wiki_index] ||= Array.new
# # update wiki index...
# wiki[:wiki_index].push(*home_page.wiki_page_references)
# end # commit changes to wiki data store file
#
# ### Some time later... ###
#
# # read wiki data...
# wiki.transaction(true) do # begin read-only transaction, no changes allowed
# wiki.roots.each do |data_root_name|
# p data_root_name
# p wiki[data_root_name]
# end
# end
#
class PStore
binmode = defined?(File::BINARY) ? File::BINARY : 0
RDWR_ACCESS = File::RDWR | File::CREAT | binmode
RD_ACCESS = File::RDONLY | binmode
WR_ACCESS = File::WRONLY | File::CREAT | File::TRUNC | binmode
# The error type thrown by all PStore methods.
class Error < StandardError
end
#
# To construct a PStore object, pass in the _file_ path where you would like
# the data to be stored.
#
def initialize(file)
dir = File::dirname(file)
unless File::directory? dir
raise PStore::Error, format("directory %s does not exist", dir)
end
if File::exist? file and not File::readable? file
raise PStore::Error, format("file %s not readable", file)
end
@transaction = false
@filename = file
@abort = false
end
# Raises PStore::Error if the calling code is not in a PStore#transaction.
def in_transaction
raise PStore::Error, "not in transaction" unless @transaction
end
#
# Raises PStore::Error if the calling code is not in a PStore#transaction or
# if the code is in a read-only PStore#transaction.
#
def in_transaction_wr()
in_transaction()
raise PStore::Error, "in read-only transaction" if @rdonly
end
private :in_transaction, :in_transaction_wr
#
# Retrieves a value from the PStore file data, by _name_. The hierarchy of
# Ruby objects stored under that root _name_ will be returned.
#
# *WARNING*: This method is only valid in a PStore#transaction. It will
# raise PStore::Error if called at any other time.
#
def [](name)
in_transaction
@table[name]
end
#
# This method is just like PStore#[], save that you may also provide a
# _default_ value for the object. In the event the specified _name_ is not
# found in the data store, your _default_ will be returned instead. If you do
# not specify a default, PStore::Error will be raised if the object is not
# found.
#
# *WARNING*: This method is only valid in a PStore#transaction. It will
# raise PStore::Error if called at any other time.
#
def fetch(name, default=PStore::Error)
in_transaction
unless @table.key? name
if default==PStore::Error
raise PStore::Error, format("undefined root name `%s'", name)
else
return default
end
end
@table[name]
end
#
# Stores an individual Ruby object or a hierarchy of Ruby objects in the data
# store file under the root _name_. Assigning to a _name_ already in the data
# store clobbers the old data.
#
# == Example:
#
# require "pstore"
#
# store = PStore.new("data_file.pstore")
# store.transaction do # begin transaction
# # load some data into the store...
# store[:single_object] = "My data..."
# store[:obj_heirarchy] = { "Kev Jackson" => ["rational.rb", "pstore.rb"],
# "James Gray" => ["erb.rb", "pstore.rb"] }
# end # commit changes to data store file
#
# *WARNING*: This method is only valid in a PStore#transaction and it cannot
# be read-only. It will raise PStore::Error if called at any other time.
#
def []=(name, value)
in_transaction_wr()
@table[name] = value
end
#
# Removes an object hierarchy from the data store, by _name_.
#
# *WARNING*: This method is only valid in a PStore#transaction and it cannot
# be read-only. It will raise PStore::Error if called at any other time.
#
def delete(name)
in_transaction_wr()
@table.delete name
end
#
# Returns the names of all object hierarchies currently in the store.
#
# *WARNING*: This method is only valid in a PStore#transaction. It will
# raise PStore::Error if called at any other time.
#
def roots
in_transaction
@table.keys
end
#
# Returns true if the supplied _name_ is currently in the data store.
#
# *WARNING*: This method is only valid in a PStore#transaction. It will
# raise PStore::Error if called at any other time.
#
def root?(name)
in_transaction
@table.key? name
end
# Returns the path to the data store file.
def path
@filename
end
#
# Ends the current PStore#transaction, committing any changes to the data
# store immediately.
#
# == Example:
#
# require "pstore"
#
# store = PStore.new("data_file.pstore")
# store.transaction do # begin transaction
# # load some data into the store...
# store[:one] = 1
# store[:two] = 2
#
# store.commit # end transaction here, committing changes
#
# store[:three] = 3 # this change is never reached
# end
#
# *WARNING*: This method is only valid in a PStore#transaction. It will
# raise PStore::Error if called at any other time.
#
def commit
in_transaction
@abort = false
throw :pstore_abort_transaction
end
#
# Ends the current PStore#transaction, discarding any changes to the data
# store.
#
# == Example:
#
# require "pstore"
#
# store = PStore.new("data_file.pstore")
# store.transaction do # begin transaction
# store[:one] = 1 # this change is not applied, see below...
# store[:two] = 2 # this change is not applied, see below...
#
# store.abort # end transaction here, discard all changes
#
# store[:three] = 3 # this change is never reached
# end
#
# *WARNING*: This method is only valid in a PStore#transaction. It will
# raise PStore::Error if called at any other time.
#
def abort
in_transaction
@abort = true
throw :pstore_abort_transaction
end
#
# Opens a new transaction for the data store. Code executed inside a block
# passed to this method may read and write data to and from the data store
# file.
#
# At the end of the block, changes are committed to the data store
# automatically. You may exit the transaction early with a call to either
# PStore#commit or PStore#abort. See those methods for details about how
# changes are handled. Raising an uncaught Exception in the block is
# equivalent to calling PStore#abort.
#
# If _read_only_ is set to +true+, you will only be allowed to read from the
# data store during the transaction and any attempts to change the data will
# raise a PStore::Error.
#
# Note that PStore does not support nested transactions.
#
def transaction(read_only=false) # :yields: pstore
raise PStore::Error, "nested transaction" if @transaction
begin
@rdonly = read_only
@abort = false
@transaction = true
value = nil
new_file = @filename + ".new"
content = nil
unless read_only
file = File.open(@filename, RDWR_ACCESS)
file.flock(File::LOCK_EX)
commit_new(file) if FileTest.exist?(new_file)
content = file.read()
else
begin
file = File.open(@filename, RD_ACCESS)
file.flock(File::LOCK_SH)
content = (File.open(new_file, RD_ACCESS) {|n| n.read} rescue file.read())
rescue Errno::ENOENT
content = ""
end
end
if content != ""
@table = load(content)
if !read_only
size = content.size
md5 = Digest::MD5.digest(content)
end
else
@table = {}
end
content = nil # unreference huge data
begin
catch(:pstore_abort_transaction) do
value = yield(self)
end
rescue Exception
@abort = true
raise
ensure
if !read_only and !@abort
tmp_file = @filename + ".tmp"
content = dump(@table)
if !md5 || size != content.size || md5 != Digest::MD5.digest(content)
File.open(tmp_file, WR_ACCESS) {|t| t.write(content)}
File.rename(tmp_file, new_file)
commit_new(file)
end
content = nil # unreference huge data
end
end
ensure
@table = nil
@transaction = false
file.close if file
end
value
end
# This method is just a wrapped around Marshal.dump.
def dump(table) # :nodoc:
Marshal::dump(table)
end
# This method is just a wrapped around Marshal.load.
def load(content) # :nodoc:
Marshal::load(content)
end
# This method is just a wrapped around Marshal.load.
def load_file(file) # :nodoc:
Marshal::load(file)
end
private
# Commits changes to the data store file.
def commit_new(f)
f.truncate(0)
f.rewind
new_file = @filename + ".new"
File.open(new_file, RD_ACCESS) do |nf|
FileUtils.copy_stream(nf, f)
end
File.unlink(new_file)
end
end
# :enddoc:
if __FILE__ == $0
db = PStore.new("/tmp/foo")
db.transaction do
p db.roots
ary = db["root"] = [1,2,3,4]
ary[1] = [1,1.5]
end
1000.times do
db.transaction do
db["root"][0] += 1
p db["root"][0]
end
end
db.transaction(true) do
p db["root"]
end
end
